For higher requirements to the operational, fire and contact safety, the network configuration of an ungrounded power supply system is used which is also referred to as an isolated network (French: Isolé Terre—IT) or as an IT power supply system. In this kind of power supply system, the active parts are separate from the ground potential—with respect to “ground”. The advantage of these networks rests upon the fact that the functionality of the connected electrical user is not impaired by an insulation fault (first fault), e.g. a ground fault or body contact, since a closed circuit cannot be formed between an active conductor of the network and ground due to the infinitely large impedance value under ideal circumstances (supply leakage capacitances are to be disregarded in this instance).
Owing to this inherent safety of the ungrounded power supply system, a continuous power supply of the user fed by the ungrounded power supply system can thus be ensured if a first insulation fault arises.
The resistance of the ungrounded power supply system against ground (insulation resistance—also an insulation fault resistance or fault resistance in a fault event) is therefore constantly monitored since a fault loop would arise via a possible other fault at a different active conductor (second fault) and the fault current flowing in this process together with an overcurrent protective device would cause a shut down of the installation resulting in an operational standstill.
Provided that the insulation state of the ungrounded power supply system is continuously monitored by an insulation monitoring device, the ungrounded power supply system can remain in operation without a predetermined time limitation even if a first fault has arisen; however, it is recommended to eliminate the first fault as soon as practicably possible.
In order to fulfil the requirements after the quick elimination of the first fault, the use of an insulation fault location system poses the state of the art in widely branched ungrounded power supply systems, in particular in extended ungrounded power supply systems, or in ungrounded power supply systems, in which a shut down of the power supply can be critical to safety.
The insulation fault location system essentially comprises a test current generator and several test current sensors mostly designed as measuring current transformers, which are connected to an insulation fault location device (insulation fault evaluation device) for evaluating the measuring signal.
If a first insulation fault has been identified in the ungrounded power supply system by the insulation monitoring device, the insulation fault location is commenced by the test current generator generating a test current and feeding this test current into the ungrounded power supply system at a central location between one or several active conductors and ground (supplied test current). A closed circuit is formed in which the test current flows from the test generator via the live active conductor, the insulation fault and via a ground connection and flows back to the test generator.
The fault location is localized via a detection of the test current in the ungrounded power supply system via the insulation fault location device using the measuring current transformers connected thereto, a measuring current transformer being specifically assigned to each cable section to be monitored. The test current detected thus corresponds to a capturable test current in the conductor section to be monitored, a negligible steady test current portion flowing outside of the fault circuit via the ohmic portions of the leakage impedances.
The respective capturable test current is captured as a differential current by all measuring current transformers, which are in the test current circuit (fault circuit) and are evaluated and indicated in the insulation fault location device. The fault location can be localized via the known assignment of the measuring current transformers to the conductor branches.
However, the capture of the test current reaches its limits because the network leakage capacitances take up non-negligible values in particular in widely branched ungrounded power supply systems and consequently (leakage) differential currents can arise which disturb the test current.
Another disadvantage of the hitherto used methods for insulation fault location is that the test current is limited to a maximum value with regard to the safety of persons and the installation. Upper threshold values in the range of 1 mA to 2.5 mA maximum are required, for example, for test currents in conjunction with sensitive installation parts—even with larger supply leakage capacitances of 10 μF to 1,000 μF.
Specifically in conjunction with high-impedance insulation faults and an amperage resulting therefrom in the fault circuit, a safe capture and evaluation of the test current and thus a reliable localization of the fault location are therefore not always ensured.
These problems are presently only met by an inadmissible exceedance of the maximally admissible test current or by a successive shut down of installation parts in conjunction with high expenditures of cost and time.